This invention generally relates to methods for heating powder materials, including processes and materials for use in the manufacturing, coating, repair, and build-up of superalloy components. More particularly, this invention relates to processes for significantly enhancing the melting of powders in a braze material by microwave energy.
Nickel, cobalt, and iron-base superalloys are widely used to form high temperature components of gas turbine engines. While some high-temperature superalloy components can be formed as a single casting, others are preferably or required to be fabricated by other processes. As an example, brazing is widely used to fabricate gas turbine components, as in the case of high pressure turbine nozzle assemblies. Brazing techniques conventionally encompass joining operations performed at an elevated temperature but below the melting point of the metals being joined. In carrying out the brazing process, an appropriate braze alloy is placed between the interface (faying) surfaces to be joined, and the faying surfaces and the braze alloy therebetween are heated in a vacuum to a temperature sufficient to melt the braze alloy without melting or causing grain growth in the superalloy base material. The braze alloy melts at a lower temperature than the superalloy base material as a result of containing a melting point suppressant such as boron. On cooling, the braze alloy solidifies to form a permanent metallurgical bond.
During engine operation, gas turbine engine components are subject to strenuous high temperature conditions under which various types of damage or deterioration can occur. As examples, erosion and oxidation reduce wall thicknesses of turbine nozzles and vanes, and cracks can initiate at surface irregularities and propagate as a result of stresses that are aggravated by thermal cycling. Because the cost of components formed from superalloys is relatively high, it is often more desirable to repair these components rather than replace them. In response, brazing techniques have been developed for crack repair and wall thickness build-up that entail placing a braze alloy filler metal on the surface area requiring repair, and then heating the filler metal in a vacuum to above its melting point, but below that of the surface substrate, so that the molten filler metal wets, flows, and fills the damaged area.
While widely employed to fabricate and repair gas turbine engine components, conventional brazing processes have notable disadvantages. First, the entire component must be subjected to a vacuum heat treatment, which is a very lengthy process in a production environment, unnecessarily exposes undamaged regions of the component to high temperatures, and can potentially remelt joints in other sections of the component. Furthermore, the braze alloy typically comprises elements similar to the base metal of the component, but with the addition of melting point suppressants (e.g., boron, silicon, etc.) that reduce its melting point below the base metal solidus temperature, thereby significantly altering its mechanical properties. Microwave brazing has been investigated as a potential candidate for eliminating these issues, as heating can be localized to selected areas of a component. Two approaches have generally been proposed for microwave brazing. A first entails the use of a susceptor (e.g., SiC enclosure) that is heated when exposed to microwave energy and, in turn, transfers the heat to the component by radiation. Drawbacks to this approach are lack of local heating of the braze alloy only, as an entire region of the component is inevitably heated, and significant heat loss from radiation in directions away from the intended brazement. A second approach entails direct microwave heating of metallic powders, which are significantly more susceptible to absorbing microwave energy than bulk metals, which reflect microwaves. However, typical braze alloy compositions do not couple sufficiently with microwave energy to be melted, with the result that the braze alloy powder is instead sintered and as a result has properties greatly inferior to the base metal of the component.